1. Field of the Invention
The present invention relates to a power supply apparatus capable of supplying stable output power when an input AC power supply fails for a short period of time and, more particularly, to a power supply apparatus having a function of diagnosing the capacity of a storage battery when the storage battery is used as part of the power supply apparatus.
2. Description of the Related Art
Uninterruptible power supply apparatuses are disclosed in various literatures such as Toshiba Review Vol. 42, No. 11 (November, 1987) PP. 877-880. Therefore, the functions and outlines of operations of the apparatuses are known. These known techniques will be summarized below.
An uninterruptible power supply apparatus constituted by a combination of a storage battery and an inverter unit is designed to supply stable AC power to a load for a predetermined period of time (e.g., 10 or 30 minutes) determined by the capacity of the storage battery even if the input AC power supply fails for a short period of time.
If an input AC power supply is assumed to fail for a long period of time or a load is a computer used for the benefit of the public, an independent power plant may be provided as backup equipment so that the independent power plant and the input AC power supply are switched to cause the independent power plant to supply AC power to the load through the inverter unit. During this power supply switching period, power supply to the load must be ensured by means of a storage battery.
As described above, the uninterruptible power supply apparatuses can continuously supply stable AC power to a load for a predetermined power failure time. Therefore, they are increasingly used as power supply apparatuses for loads requiring high reliability, such as large scale computers.
The backup time, however, upon a power failure of an input AC power supply in an uninterruptible power supply apparatus is determined by the capacity of a storage battery. The capacity of the storage battery is initially selected depending on a system capacity (the load capacity of the uninterruptible power supply apparatus). However, the storage battery tends to be degraded in characteristics over time, and is changed in discharging capacity depending on an operation temperature and the like.
In the conventional uninterruptible power supply apparatus, therefore, whether the storage battery can cover a power failure backup time required by a system is determined by causing the storage battery to discharge when the input AC power supply actually fails. For this reason, a downward tendency of the capacity of the storage battery cannot be detected unless a shortage of a power failure backup time actually occurs. In addition, if such a trouble occurs, this causes a lot of inconvenience to many and unspecified users of the load (computer system) (e.g., interruption of an online system in a bank or interruption of operations at a counter of an air line). That is, the drawback of the conventional power supply apparatus as a power supply apparatus requiring high reliability is that the user is always attacked by an indefinite inquietude.